1. Field of the Invention
The present invention relates to a transmission circuit provided in a communication terminal and, more particularly, to a transmission circuit using the HPSK (Hybrid Phase Shift Keying) modulation scheme.
2. Description of the Prior Art
In the HPSK modulation scheme described in 3G TS 25.213 of 3GPP (3rd Generation Partnership Project) which is a standardization project for W-CDMA (Wide band Code Division Multiple Access) specifications, transmission data is spread by a spreading code first, and then the spread transmission data is multiplied by a gain factor to perform amplitude weighting in HPSK modulation so as to obtain amplitude data. Thereafter, this amplitude data is HPSK-modulated.
FIG. 1 is a block diagram showing an example of the arrangement of a conventional transmission circuit using the HPSK modulation scheme.
As shown in FIG. 1, this conventional circuit is comprised of a baseband circuit 110 for generating and outputting two types of transmission data, namely data channel data DPDCH (Dedicated Physical Data Channel) and control channel data DPCCH (Dedicated Physical Control Channel), a multiplier 120 for spreading the data channel data DPDCH output from the baseband circuit 110 by multiplying the data channel data DPDCH by a spreading code SCd, and outputting the resultant data as spread data d, a multiplier 122 for spreading the control channel data DPCCH output from the baseband circuit 110 by multiplying the control channel data DPCCH by a spreading code SCc, and outputting the resultant data as spread data c, a multiplier 121 for outputting amplitude data Iin by multiplying the spread data d output from the multiplier 120 by a gain factor βd, a multiplier 123 for outputting amplitude data Qin by multiplying the spread data c output from the multiplier 122 by a gain factor βc, an HPSK modulation circuit 130 for receiving the amplitude data Iin and Qin respectively output from the multipliers 121 and 123 and outputting HPSK-modulated data Iout and Qout by mapping the input amplitude data Iin and Qin on the complex I-Q plane in accordance with a scrambling code which is one of the frequency spreading codes in the CDMA scheme and output from the baseband circuit 110, a digital filter 140 for removing high-frequency components from the HPSK-modulated data Iout output from the HPSK modulation circuit 130 and outputting the resultant data as a digital signal Id, a digital filter 142 for removing high-frequency components from the HPSK-modulated data Qout output from the HPSK modulation circuit 130 and outputting the resultant data as a digital signal Qd, a digital/analog converter 141 for converting the digital signal Id output from the digital filter 140 into an analog signal Ia and outputting it, a digital/analog converter 143 for converting the digital signal Qd output from the digital filter 142 into an analog signal Qa and outputting it, and a quadrature modulator 150 for outputting an HPSK signal having a desired frequency by quadrature-modulating the analog signals Ia and Qa respectively output from the digital/analog converters 141 and 143.
Note that each of the spreading code SCd by which the data channel data DPDCH is multiplied by the multiplier 120 and the spreading code SCc by which the control channel data DPCCH is multiplied by the multiplier 122 is one of the frequency spreading codes in the CDMA scheme and has a rate equal to the chip rate. These codes differ for the respective transmission channels to maintain orthogonality between the channels and are output from the baseband circuit 110.
The gain factor βd by which the spread data d is multiplied by the multiplier 121 and the gain factor βc by which the spread data c is multiplied by the multiplier 123 are unique to HPSK modulation. These gain factors are values for respectively weighting an I (Inphase) amplitude and Q (Quadrature) amplitude and output from the baseband circuit 110. Each of the gain factors βd and βc takes a value from 0 to 15 depending on the transmission data rate. One of the gain factors βd and βc is always “15”. In addition, since the control channel data DPCCH is always required, the gain factor βc will never be “0”.
The amplitude data Iin and Qin respectively output from the multipliers 121 and 123 are obtained by converting the values of “0”/“1” of spread data d and c respectively output from the multipliers 120 and 122 into amplitude values with positive and negative signs and expressed by binary codes in two's complement form.
In the transmission circuit having the above arrangement, the data channel data DPDCH and control channel data DPCCH output from the baseband circuit 110 are respectively multiplied by the spreading codes SCd and SCc to obtain the spread data d and c, and the amplitudes of the spread data d and c are respectively weighted by the gain factors βd and βc, thereby performing HPSK modulation.
High-frequency components are removed from the HPSK-modulated data Iout and Qout, and the resultant data are converted into analog signals. Thereafter, the signals are quadrature-modulated, and the resultant data is output as an HPSK signal having a desired frequency.
In the above transmission circuit, however, since the values of gain factors by which spread data are multiplied are directly reflected in the amplitudes of the HPSK-modulated signals on the complex I-Q plane, the output power of the quadrature modulator changes as the combination of gain factor changes. If the output power of the quadrature modulator changes, the S/N ratio varies. As the output power decreases, the S/N ratio decreases, resulting in a deterioration in adjacent channel leakage power characteristic.
In a system using the CDMA scheme, it is required to always keep the power of the control channel data DPCCH at the antenna end constant even with a change in data rate if the communication condition at the terminal, e.g., the distance between the terminal and the base station, remains the same. In the conventional transmission circuit described above, however, the power of the control channel data DPCCH at the antenna end cannot be kept constant depending on a change in the combination of gain factors or the output power of the quadrature modulator.